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Brain Worm

brain worm adult on moose's spinal cord
Adult brain worm (P. tenuis) on the spinal
cord of a moose submitted for diagnosis.
Photo by DEC's Wildlife Health Unit

Other Names: "moose sickness", Parelaphostrongylus tenuis (P. tenuis), meningeal worm, cerebrospinal nematodiasis, cerebrospinal parelaphostrongylosis.

Brain worm is the term commonly applied to the parasitic nematode (round worm), Parelaphostrongylus tenuis (P. tenuis). White-tailed deer are the normal host for this parasite. Most of the time, they are not affected by the parasite. However, other species such as moose, mule deer, reindeer/caribou, sheep, goats, alpacas, and llamas are abnormal hosts and can develop disease or die if infected.


Typically, most white-tailed deer do not show any signs because they are the normal host. Abnormal hosts (moose, mule deer, elk, caribou/reindeer, llama, alpaca, goat, or sheep) may display the following:

  • ataxia (unsteady gait, loss of voluntary muscle control)
  • listlessness
  • general weakness
  • fearlessness
  • apparent deafness and/or blindness
  • circling
  • moose showing signs of brain worm
    A cow moose (adult female) exhibiting neurologic signs
    of brain worm in Rensselaer County, NY.
    Photo by DEC's Wildlife Health Unit
  • unusual head tilt or neck position
  • inability to feed/forage
  • emaciation
  • paralysis


Definitive diagnosis relies on the detection of adult P. tenuis in the spinal cord or brain during necropsy (animal autopsy).

Presumptive diagnoses are sometimes based on gross or microscopic evidence of worm damage in the spinal cord or brain. Clinical signs (listed above) are considered, especially if a necropsy cannot be conducted.

Life Cycle

brain worm life cycle
Illustration by Natalie Sacco, NYSDEC

Life Cycle Description in a Deer

  • Deer accidentally consume an infected gastropod (snail or slug) while feeding.
  • Larvae in the gastropod penetrate the deer's stomach wall and travel along the nerves of the deer until it reaches the spinal cord and moves into the brain.
  • In the brain, the larvae mature into an adult and prepare to reproduce.
  • Adult worms lay eggs on the dura mater (the outermost of the three layers of the meninges surrounding the brain and spinal cord) or directly into the bloodstream of the deer.
  • Larvae hatch and enter the deer's bloodstream. They travel to the lungs, where they are coughed up the windpipe. Once in the mouth, they are swallowed and pass through the digestive tract.
  • The larvae are then excreted with the fecal pellets. The larvae (first stage) are now in the mucus covering the deer's fecal pellets.
  • Gastropods often feed on the mucus on the fecal pellets. The larvae penetrate the gastropod's foot (part of the snail or slug that is used for locomotion). Once in the gastropod, the larvae mature into second and third stage larvae that are infectious to a new host.

Life Cycle Description in a Moose

  • The infected gastropod is inadvertently consumed by a moose.
  • The parasitic larvae travel to the spinal cord and brain of the moose, as it does in the deer.
  • The nematode disrupts the nervous tissue through mechanical destruction, manipulation, and/or inflammation. Several days after a moose is infected, it may have neurologic problems or abnormal behavior.
  • After infection, there may be periods where the moose seems to recover as the worm or worms move through different portions of the brain or spinal cord.
  • An adult P.tenuis within the brain or spinal cord of moose can be fatal. Death can be the result of:
    • paralysis;
    • lack of fear/inappropriate behavior (resulting in motor vehicle strike or being shot by police or Environmental Conservation Officer); or
    • inability to feed (starvation) or feeding on inappropriate food items (malnutrition).

Management Implications

Brain worm is common in New York's white-tailed deer population, and most of the time they suffer little consequence. Brain worm is often fatal for other species. Affected moose cause public concern due to their abnormal behavior. Environmental Conservation Officers, DEC Biologists, or local Law Enforcement should be called to the scene to evaluate a sick moose. Ill moose may be euthanized and submitted to DEC's Wildlife Health Unit for necropsy and testing, which usually includes brain worm, chronic wasting disease, and rabies.

Currently, there is a multi-institution study looking at New York moose. If you spot a moose in the wild, we are interested in hearing about it.